A hot-dip zinc alloy coated steel sheet has been used for various purposes, but a welded zinc alloy coated steel sheet may have a problem that cracks are formed in a welding heat affected zone. This phenomenon is generally referred to as “liquid metal embrittlement cracking” and is considered to occur since the molten coating component acts on the grain boundary of the steel sheet to cause brittle fracture (grain boundary fracture).
In the zinc alloy coated steel sheet, a hot-dip Zn—Al—Mg alloy coated steel sheet is used in various purposes requiring corrosion resistance, such as a building material, due to the excellent corrosion resistance thereof. In recent years, a hot-dip Zn—Al—Mg alloy coated steel sheet has been increasingly applied to an alternative to an ordinary galvanized steel sheet. However, the hot-dip Zn—Al—Mg alloy coated steel sheet has a higher tendency to suffer liquid metal embrittlement cracking than the ordinary hot-dip galvanized steel sheet.
As a measure for preventing the liquid metal embrittlement cracking, it has been known that a base sheet for coating containing B is effectively used (see PTL 1).